A tire for a vehicle typically includes a tire carcass, breaker belt and tire tread. For example, the tire carcass for a radial tire includes radial plies extending from bead to bead and embedded in rubber. The radial plies generally include at least two layers, wherein each layer has a series of reinforcing cords which extend substantially perpendicular to the circumferential direction of the tire. Further, the belt plies for the radial tire are positioned between the carcass and the tire tread, and also generally include at least two layers having reinforcing cords.
The tire tread extends around the outer circumference of the tire and is designed to contact the road surface. The tire tread has a pattern which is segmented into a plurality of raised, load-supporting blocks separated by intersecting circumferential and transverse grooves and/or sipes. The blocks determine the control, acceleration and braking characteristics of the tire, while the grooves and/or sipes are necessary to provide flexibility and water removal.
The tread blocks for the tire are typically arranged in "base pitches", wherein each base pitch includes a predetermined geometry of whole and/or partial tread blocks. Each base pitch may vary in width across the tire. For example, each base pitch can extend transversely from the shoulder to the center line of the tire on each circumferential section or "mold half" of the tire. Alternatively, each base pitch can extend transversely over only a single circumferential section of the tire, for example over a single rib or combination of ribs. In any case, the base pitches are repeated around the circumference of the tire to form the complete tire tread.
The constant contact of the tire tread on the road surface can in some instances produce unwanted noise. In particular, as the tire contacts the road surface, the individual tread blocks cause air disturbances upon impact with the road, creating a spectrum of audio frequencies broadly referred to as "tire noise". Tire noise is generated at least in part by: (1) the impact of the tread block on the road surface; (2) the vibration of the tire carcass; and (3) the "air pumping" which occurs as the tread blocks become compressed and expand into the grooves separating adjacent blocks. The fundamental frequency of the noise is a function of the number of tread blocks around the circumference of the tire and the rotational speed of the tire.
Techniques have been developed to distribute the noise frequency produced by the tire tread over a wide frequency band to approach what is termed "white noise". For example, one known technique for reducing tread noise is to use base pitches having different pitch lengths, wherein the "pitch length" is a measure of the length from the leading edge of one base pitch to the leading edge of the next adjacent representative base pitch in the circumferential direction of the tire. A plurality of base pitches having different pitch lengths is conventionally referred to as a "pitch sequence". Other techniques use only random or sinusoidal sequencing of the pitches in an attempt to modulate the objectionable noise producing frequencies.
For example, Vorih, U.S. Pat. No. 3,926,238, discloses a technique for modulating noise produced by rotating load carrying elements by providing discrete pitch lengths in which the ratio of pitch lengths falls within the open intervals defined by the boundary nodal points (N-1)/N:1.0, (N being an integer selected from 2, 3, 4, 5 or 6), and excluding the ratios defined by the boundary nodal points. Further modulation is obtained by sequencing the individual pitches in such a manner that the sequence is characterized by a plurality of strings of the load carrying elements, each string consisting of at least three consecutive load carrying elements of substantially identical pitch length. The length and sequential positioning of the strings are selected to modulate the block frequency harmonic which is produced upon rotation of the tire.
The above techniques, however, are primarily designed to reduce noise in tires by varying the pitch length of the tread blocks around the circumference of the tire to spread the noise over a broad frequency band. The techniques are not directed toward reducing tire noise by selectively arranging the pitch sequence in one circumferential section of the tire relative to the pitch sequence in at least one other circumferential section.
One technique for reducing tire noise in tires by selectively arranging groups of pitch sequences in circumferential sections is shown in Lippmann et al, U.S. Pat. No. 2,878,852. Lippmann discloses a tire tread having male and female mold halves, wherein each mold half has a pitch sequence comprising individual "design units" i.e., a preselected number of tread blocks, extending around the circumference of the tire. The circumferential length of the repeating design units of the tread pattern are selected to be mirror images of themselves or of other groups of design units in each respective mold half. The pitch sequence in the male mold half, however, is identical to the pitch sequence in the female mold half. Accordingly, the Lippmann technique is not directed toward situations where the pitch sequence in one circumferential section (or mold half) is different then the pitch sequence in another circumferential section (or mold half). Moreover, the Lippmann technique does not selectively arrange the pitch sequence in each mold half relative to each other to reduce tire noise.
Accordingly, although the above-identified references disclose certain techniques for reducing tire noise, there is a constant demand in the industry for improved techniques which further reduce the tire noise, and in particular, for techniques which reduce the tire noise in tires having pitch sequences arranged in at least two circumferential sections, regardless of the pitch sequences employed.